Don't I remember that one of the few things Rich Measures said, that
others agreed with was, that the SB-220 filament transformer inherently
limits inrush current to a safe level?
73, Pete N4ZR
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On 6/16/2012 9:02 AM, Jim Thomson wrote:
> Date: Sat, 16 Jun 2012 10:38:47 +0930
> From: "Leigh Turner" <invertech@frontierisp.net.au>
> Subject: Re: [Amps] filament instant heating query / simple remedy
>
> All good points you make here Jim.
>
> When it comes to the small desktop amps like the SB220 and TL922 having
> separate filament transformers I find that inserting a pair of GE CL-60 NTC
> thermistors in each leg of the filament transformer primary winding provides
> a simple remedy for implementing a soft-start. The observed glow of the two
> 3-500Z filaments comes up nice and slow as the thermistors warm up...no big
> surge in brightness at initial turn on as it was with the stock design.
>
> This is in addition to a simple short-duration resistor / AC relay coil
> style step-start to both the HV and filament xfmrs timed predominantly from
> the HV electrolytic charge up time constant. This relieves the inrush
> current surge stress on the front panel ON/OFF switch contacts and the
> electrolytic capacitor bank.
>
> Cheers,
>
> Leigh
> VK5KLT
>
> ## I also looked into the thermistors... since RF parts sells em just for
> this tube
> fil application. In normal operation, they run hot all the time though. You
> cant
> cool em with any air either, or the resistance will increase on em. Since
> the HV supply
> needs some step start anyway.... you can kill 2 x birds with one stone ! A
> simple
> 25 ohm resistor in one leg of the 240 line works wonders. Fil brilliance is
> aprx
> .66 of normal. Once the resistor is shorted out, tube brilliance comes up
> to
> normal..albeit instantly. I like your thermistor idea though, since it
> brings
> the tubes up slower. I already have step start for the hv and fil xfmr...
> so à
> adding just one thermistor into one leg of the fil xfmr is no big deal. I
> will
> get a bunch, and implement it into the 3 x L4B’s I have. Do you need
> one thermistor per leg... or just one period. How long is the slow ramp up,
> 3 secs, 10 secs or what.
>
> ## I see no reason why thernistors couldn’t be used on my 3CX-3000A7 fil
> xfmr...
> provided they are sized correctly of course. I use a variac anyway, used
> in conjunction
> with a sola constant v xfmr... to precisely set the fil V..and to slowly
> ramp up the fil.
> Thermistors would allow me to leave the variac alone, and just use it to
> set fil V.
>
> tnx...... Jim VE7RF
>
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